The pilot of the aircraft also needs navigation information allowing him to follow a flight plan. Amongst this information, the heading allows him to direct the aircraft with respect to magnetic North. The heading may be obtained on the basis of a magnetic sensor generally situated far from the instrument panel. Indeed, this sensor, measuring the terrestrial magnetic field, may be disturbed by the aircraft itself and notably by the onboard instruments which generate electromagnetic disturbances. These instruments being numerous in the instrument panel, it is sought to move the magnetic sensor further away, by placing it outside the cockpit of the aircraft for example.
A course compass is also provided as stand-by in the cockpit of the aircraft so as to alleviate any defect in the primary information chain presenting the heading information to the pilot. This stand-by compass is commonly placed in the top part of the canopy of the cockpit, this location being furthest from the other onboard instruments, therefore the least disturbed by them.
Although the magnetic sensors are placed in the locations that are as little disturbed as possible, a magnetic deviation persists that it is necessary to compensate. For this purpose, a compensation procedure is provided, making it possible to correct the effects of the aircraft and of all the instruments that it comprises. This procedure makes provision for example to measure the direction of the magnetic field by means of the sensor to be compensated for during angular displacements of the aircraft on an aerodrome runway. The measurements are thereafter compared with the terrestrial magnetic field of the aerodrome so as to establish a deviation curve that it will be possible to use in flight to compensate for the disturbances on the sensor. Accordingly, a specific instrument is connected to the sensor, the former making it possible to directly recover the measurement made by the sensor before this measurement is processed to obtain a display readable by the aircraft pilot. The magnetic sensor associated with the primary instruments as well as the stand-by compass may be compensated.
Moreover, it has been attempted to use the stand-by instrument to determine and display the heading followed by the aircraft. This heading may be used when the aircraft's primary instruments have failed as for example described in patent application FR 2 937 415. This document describes the use of magnetic sensors internal to the stand-by instrument to determine and display the heading followed by the aircraft. This document also describes a calibration procedure using the stand-by instrument itself and its display screen. This calibration procedure is beneficial since it does not require any specific instrument having to be connected to the stand-by instrument for the processing of the data gathered. In practice this procedure is usable only under conditions where the aircraft's primary instruments are operating correctly. Indeed the calibration procedure described in document FR 2 937 415 makes complete use of the screen of the stand-by instrument during calibration. During calibration it is impossible to access the basic information delivered by the stand-by instrument such as the altitude, the speed and the attitude of the aircraft. For example, in flight, when the primary instruments have failed, the pilot uses the stand-by instrument to ascertain the flight information necessary for piloting. If magnetic calibration of the stand-by instrument is necessary, this calibration deprives the pilot of all flight information during the calibration, which may be prejudicial to flight safety.
In civilian aeronautics, a procedure exists which makes provision for a flight during which the aeroplane undergoes acceptance testing. During this flight, all the instruments must be operational, the primary instruments and the stand-by instruments. Consequently, it is impossible to use this flight to perform the calibration of the magnetic sensors internal to the stand-by instrument. The use of another flight for calibration entails additional costs for the aircraft manufacturer.